Parallax occlusion portal effect
One way si to build both worlds inside one scene.
And then fake an 'infinitely deep' wall between them.




Build one set (I used an HDRI) 
Add a wall with a 'portal' cutout.
Add a texture to this wall (I used stars) and use Texture Coordinate node > Window to map it.
The texture will always be facing towards you, faking an infinitely deep space.


Since both worlds are actually inside one scene, you can freely move objects between them.

I'm not rotating the portal, I'm moving the camera.